The present invention relates to a document exposure unit with which a copying apparatus is equipped, and more particularly to a document exposure unit wherein, for the purpose of preventing a platen glass from being overheated excessively through illumination by an exposure lamp, a cooling fan for preventing overheating is caused to start running after a certain period of time from the start of copying and thereby the cooling fan is operated most efficiently in terms of operation time or it is operated efficiently through judgment of operation conditions in the copying apparatus, and thus an optical system including a platen glass is prevented from being soiled with dust in the air or with toner.
In general, a powerful light source is used in a copying apparatus for the exposure of a document, and a light source with an intensity of light as high as several hundred watts is usually used. After making several hundred copies, therefore, the temperature of the platen glass on which a document is placed goes up sharply as shown by curve A in FIG. 4, reaching around 70.degree. C., which causes the problem that it is difficult for an operator to place a document on the platen glass and that the platen glass tends to be broken due to its high temperature. For preventing such problems, there has been known a method to prevent the temperature rise on a platen glass by reducing the copy speed. In this method, however, copying efficiency is lowered. In order to improve copying efficiency, therefore, there has been used a method to cool the bottom surface of the platen glass forcibly, in which a method to cool with a fan is most economical and efficient in general. However, when a fan is caused to continue running during copying operations regardless of the temperature of the platen glass, toner, especially black toner used generally scattered from a developing unit provided in a copying apparatus is sucked by the fan and adheres to the bottom surface of the platen glass and to the reflection mirrors used in an optical system. With an increase in copying operations, therefore, even when the surface of a document is exposed to an exposure lamp, clear and sharp copy images can not be obtained due to an insufficient amount of light. In a method disclosed in Japanese Utility Model Publication Open to Public Inspection No. 10752/1989 (hereinafter referred to as Japanese Utility Model O.P.I. Publication) as a means for solving such a problem, the temperature on the surface of a platen glass is measured by a detecting device during continuing copying operations, and when the temperature exceeds a certain level, a fan for cooling is operated. This method, however, requires a detecting device for the temperature, resulting in a complicated structure and high cost of the apparatus.
Since a fan employs a motor generally, it is possible to rotate it at high speed by boosting voltage or increasing a current to rapidly cool a platen glass, an exposure lamp and optical mirrors. In that case, however, the aforementioned suction of toners is increased and consequently, scattered toners are strongly blown against the platen glass, the exposure lamp and the optical mirrors, resulting in contamination of the optical mirrors and an insufficient amount of light from the exposure lamp for practical use. When a fan is rotated at low speed, on the contrary, the cooling effect is lowered accordingly. One way for coping with this is to lower the copying speed. This method, however, reduces the function of a copying apparatus. As described above, it is very difficult to control the rotation of a cooling fan, and the temperature rise varies complicatedly depending upon the mode of copying operations, such as sheet size, magnification change and the number of copies. In a conventional method in the past, the cooling fan has been constantly operated during copying operations without coping with variation in temperature rise mentioned above, resulting in the platen glass and mirrors both becoming contaminated with toner and dust, and in inefficient cooling.